Single photons, or photons of which the number included in a pulse is limited to one, have recently been attracting attention in basic research fields related to quantum mechanics and in applied research fields such as absolutely secure quantum cryptographic communication with eavesdropping detection (see Japanese Patent Application Laid-Open No. 2009-147460 (Patent Literature 1)). In particular, long distance single photon transmission in 1.3-μm and 1.55-μm wavelength bands, which are low-loss regions (telecommunication wavelength band) of optical fibers, are needed for the practical use of quantum cryptographic communication.
Conventionally, as a single-photon generation source in the telecommunication wavelength band, a laser light source is simply attenuated to obtain a single photon. However, the low generation efficiency of the single photon causes a significant decrease in transmission distance and communication speed. A single-photon light source that generates a single photon with high efficiency and high reliability is thus desired to be constructed.
As materials for achieving the generation of a single photon, compound semiconductor quantum dots, defects in diamonds (NV centers), and the like have been reported. There has been observed an antibunching behavior which is evidence of suppression of simultaneous photon generation during the generation of a single photon (see M. J. Holmes et. al., “Room-Temperature Triggered Single Photon Emission from a III-Nitride Site-Controlled Nanowire Quantum Dot,” Nano Lett. 2014, 14, 982-986. (Non-Patent Literature 1), K. Takemoto et. al., “Non-classical Photon Emission from a Single InAs/InP Quantum Dot in the 1.3-μm Optical-Fiber Band,” Japanese Journal of Applied Physics Vol. 43, No. 7B, 2004, pp. L993-L995. (Non-Patent Literature 2), I. Aharonovich et. al., “Diamond-based single-photon emitters,” Rep. Prog. Phys. 74 (2011) 076501 (28pp). (Non-Patent Literature 3)).
Single-photon sources in the telecommunication wavelength band are currently constructed by using InAs or other compound semiconductor quantum dots. Such single-photon sources are only operable at extremely low temperatures like 10 K, and thus require cooling by liquid helium which is expensive and rare resources.
The generation of a single photon at room temperatures is achieved by using CdSe, GaN, or other compound semiconductors or diamond NV centers. In any of these, the emission wavelength is in the visible range, and the generation of a single photon in the telecommunication wavelength band at room temperatures has not been reported.
An exciton is a pair of electron and a hole bound to each other. Carbon nanotubes have exciton binding energy of approximately several hundreds of milli-electron volts, which is approximately 10 times that of conventional solid semiconductors. This allows the stable presence of an exciton at room temperatures.
Localized excitons or confined excitons can be formed to limit the number of excitons in a carbon nanotube to one through discretization of levels as with a quantum dot and through annihilation of excitons. Such a single exciton, when relaxes, generates a photon. This can be utilized to generate a single photon, an only photon included in a light emission pulse.
Carbon nanotubes are known to emit light in a near infrared range of approximately 0.8 μm to 2 μm in wavelength, depending on the chirality which indicate the structure of the carbon nanotubes and diameter. In particular, carbon nanotubes emit light in the telecommunication wavelength band which is low-loss ranges of optical fibers (1.3 μm and 1.55 μm in wavelength). Carbon nanotubes are thus expected to generate a single photon having a wavelength in the telecommunication wavelength band. At present, the generation of a single photon in the telecommunication wavelength band from a carbon nanotube has not been reported.
It has heretofore been reported that a carbon nanotube produced a single photon from a localized exciton at temperatures of 50 K or lower (see A. Hoegele et. al., “Photon Antibunching in the Photoluminescence Spectra of a Single Carbon Nanotube,” PRL 100, 217401 (2008). (Non-Patent Literature 4)). However, the produced single photon was not in the telecommunication wavelength band. There has been no report of a single photon obtained at high temperatures above 50 K, including room temperatures.